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Comment by ceejayoz | Hacker News Reader

Initially, I think you're right.

Eventually, I think they're aiming for airliner-style reuse. Engineer any frequently-breaking parts to be better, then check every X flights once they've got the kinks worked out.

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I am guessing it will be more like an engine rebuild on an engine for a 777 - tear down and xray of key parts. The turbopumps and compressors in a rocket engine are very sensitive, high precision parts.

> The turbopumps and compressors in a rocket engine are very sensitive, high precision parts.

SpaceX deliberately engineered lower performance, higher reliability engines. Think minivan instead of Ferrari. The SSMEs were genuine marvels of engineering, but that came at significant cost to reusability.

Also I think 2 of them can flame out and Falcon can still achieve the primary mission. Minivan with an extra tire.

That depends very much on what the primary mission is. For LEO missions like the CRS flights, then sure. For GTO launches, they probably don't have the margin needed.

For a GTO mission the Falcon 9 can have a single engine out and still complete the primary mission.

Is this still true at max GTO payload weight?

Remember that with reusability in play, the guidance system can decide "screw the first stage landing, the payload is more important" and simply switch to an expendable launch profile if it turns out that you have more engine problems on the first stage than you designed for. Then you use the reserve fuel to get the $300M satellite safely to orbit instead of getting the $30M stage safely to the ground. The customer is happy and you've only lost a unit for which you have a dozen of direct substitutes in storage anyway (SpaceX already doesn't have enough storage space, apparently!).

It might depend where in the flight the engine fails, assuming we're talking about the first stage. If I had to guess, a GTO mission (placing a payload in a 36,000 km x 350 km elliptical orbit) could still be successful with an engine failure in the first stage after 65-75% of the fuel had been burned, and the vehicle was generally lighter, allowing the burn time to be adjusted longer to reach the same velocity at the point of first stage cutoff and separation to ignition of the second stage. But this guess comes from dozens of hours spent playing Kerbal Space Program....

Or a minivan with runflat tires. They'll still get you where you're going, but a bit slower and not quite as efficiently.

Isn't that the kind of detailed tear down that forced the shuttle in to being more expensive than traditional systems?

Indeed. Hopefully SpaceX can do less, since their hardware is more robust. The Space Shuttle engines were driven at the edge of failure because they needed every last bit of performance. SpaceX's designs tend to go for robustness at the cost of efficiency, so they ought to need less work.

Possibly of interest, the giant engines used in the Delta IV Heavy have 80% less part count than an SSME, but they were never intended to be human-rated: https://en.wikipedia.org/wiki/RS-68

Not that it has brought launch costs down much, considering how the ULA loves their juicy government contracts for billion dollar NSA/NRO satellite launches.

IIRC the Falcon 9 engine design is the opposite of an RS-68, which is the "one huge fucking engine" design philosophy, which is why a falcon9 has a cluster of many smaller engines and is tolerant of an individual engine failure during launch.

> IIRC the Falcon 9 engine design is the opposite of an RS-68, which is the "one huge fucking engine" design philosophy, which is why a falcon9 has a cluster of many smaller engines and is tolerant of an individual engine failure during launch.

Indeed! A Falcon 9 mission has lost an engine mid-flight, and the flight system adjusted the burn duration on the fly to compensate:

http://motherboard.vice.com/blog/spacex-s-non-fatal-failure

Also, every shuttle flight is human-rated. I would guess that SpaceX wouldn't use "refurbished" engines for manned flights, while they could cut down on the thoroughness of checks for lower value payloads.

That seems likely at first, but it'll be really exciting when that switches around and reused engines are the preferred choice for manned flights since they've been proven.

After all, you never put passengers on the very first flight of a new airliner. You test it out first, and then once it's demonstrated you start using it for real. We'll know that reusable rockets have really made it when the first flight of a new one is done as a test flight without risking any lives, and only afterwards is it put into normal use.

Plus for the orbiter refurb they had to check the entire heat shield, flight control systems, crew habitats, etc. etc. Easier when it's just the rocket + fuel tank.

Indeed. The heat shield in particular was extremely delicate and tended to get beaten up a lot coming back, so that was a huge amount of work to get ready for the next flight.

The entire spaceplane concept drastically complicated things. Capsule-based return systems are vastly simpler to operate, which is despite everything the Soyuz has been flying continuously this whole time.

Downside is you can't capture enemy satellites and return them to ground for study/reverse engineering.

This is how NASA got many billions of funding from the USAF for part of the space shuttle program - look through the list of flights in the 1980s where the payload was an unspecified national security satellite. The capability was promised and it was funded for it - but almost never used. The actual ability to grab a big thing and return it was hardly ever used, such as the LDEF mission. It doesn't help that a huge percentage of Soviet satellites that they might want to have grabbed would have been unreachable from a space shuttle launch site (IMINT satellites in 90 degree inclination polar orbits, satellites in molniya orbits, radar naval observation satellites in MEO orbits) all beyond the orbit/delta-V capabilities of a shuttle mission. And of course geostationary orbit where no human has ever gone.

The first few, probably yes.

But I'm pretty convinced they're trying to avoid this. The engines have been test-fired many, many, times (30 starts is a figure that has been mentioned a few times. Remember that the first recovered stage (Orbcom) was returned to the launch pad and went through a static fire just a few weeks after landing. It seems clear to me that they've got their eye on reasonably-quick-turnaround reuse.

I bet they're less concerned with # of firings and more with fatigue/wear from atmosphere reentry. Very hard to test on ground.

They've now got 45 intact engines back, from a variety of re-entry regimes. We know they've done an all-up static fire. Several of the returned stages have had engines removed and I imagine some of these have been tested.

If there's a major problem, they already know about it.

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